Authorities discovered a Batman mask in the apartment of James Eagan Holmes, the suspected killer of a dozen people in Aurora, Colo., a law-enforcement official told the Associated Press late Sunday night.

Although the official spoke on condition of anonymity because he was not authorized to speak to the press, this is not the first report to indicate that the timing of the shooting rampage — which killed 12 and injured 58 people during a midnight premiere of The Dark Knight Rises — was not entirely random. The New York Daily News reported that police discovered a Batman poster in Holmes’ apartment, and New York City Police Commissioner Raymond Kelly said the suspect identified himself as “the Joker” after his arrest. Still, these details can only hint at a motive, and Aurora Police Chief Dan Oates said it could take several months for authorities to develop a personality profile for Holmes.

One Colorado gun-range owner had firsthand experience with the suspect’s fascination with the comic-book character. The alleged killer applied for membership at the Lead Valley Range in Byers, Colo., last month, but the club’s owner, Glenn Rotkovich, told ABC News that he rejected Holmes because he thought he was “creepy.” Rotkovich said he got a “bizarre” Batman-inspired voice-mail message from Holmes that led him to issue a clubwide ban on the 24-year-old.